1. Field of Invention
The present invention relates to a zoom lens, particularly of the mechanical compensation type.
2. Description of Prior Art
Quite recently, the magnification ratio of zoom lenses used with compact motion picture cameras have been considerably increased. However, a zoom lens with high magnification factor becomes unavoidably large and heavy with inferior operability so that a compact lens is desirable. Under such circumstances, attempts have been made to make the whole system compact in the state while the given focal length and the given F number are kept unchanged by increasing the refractive power of each group constituting the zoom lens so as to make the travel of the movable lens group small during the zooming operation. This would minimize the increase in the total length and the diameter.
However, these refractive-index increases result in difficulty in achieving aberration correction and require severe control of the manufacturing accuracy in the design and the manufacture of the zoom lens. Ultimately, if is disadvantageous for a practical zoom lens of high quality. Below, the difficulties of realizing a compact lens will be explained in detail in accordance with embodiments of a conventional zoom lens.
Table 1 shows data concerning the zoom lens mentioned as the first embodiment in U.S. Pat. No. 3,972,591 and having a focal length 7.238-68.79 and F Number 1:1.4. The data shows focal lengths and the distance between the principal points of the respective groups.
TABLE 1 ______________________________________ Distance between principal points Lens group Focal length f = 7.238 f = 15.72 f = 68.79 ______________________________________ 1 61.153 8.79 22.616 36.442 2 -15.120 79.38 58.365 23.287 3 35.716 5.226 12.415 33.667 4 -49.989 37.23 37.23 37.23 5 26.586 ______________________________________
In this zoom lens, the first, the second the third and the fourth groups constitute an afocal portion. Decreasing the afocal zoom part proportionally and keeping the relay lens group unchanged, produces a zoom lens whose focal length range for the whole system is same whose linear scale for the zoom part is decreased. When the figures for the first, the second, the third and the fourth groups among the data in Table 1 are multiplied by 0.8, a compact lens shown in Table 2 is realized.
TABLE 2 ______________________________________ Distance between principal points Lens group Focal length f = 7.238 f = 15.72 f = 68.79 ______________________________________ 1 48.9224 7.032 18.0928 29.1536 2 -12.096 63.504 46.692 18.6296 3 28.5728 4.1808 9.932 26.9336 4 -39.9912 37.23 37.23 37.23 5 26.586 ______________________________________
In order to learn quantitatively the extent of the difference between the lenses in accordance with Tables 1 and 2, the height h.sub.1 at which the paraxial principal ray passes through the principal plane of the first group is calculated for the short focal length as well as for the long focal length, while .sub.i .SIGMA..sub.1 1/fi is obtained as the factor for comparing the Petzval sum thereby. The value .alpha..sub.1 of the angle at which the paraxial principal ray is incident on the first group at the end of the wide angle side in the state in which the lens is focused at the infinite distance is taken 1, the whole system is normalized with the focal distance 7.238 at the end of the wide angle side, the aperture is arranged at the distance 13.68 mm behind the fourth group and the object is at the distance 1.2 m.
The height .eta. at which the largest field angle principal ray is incident on the principal plane of the first group when the aberration and the distortion are eliminated is EQU .eta.=h.sub.1 .times.ymax
wherein ymax is the largest ideal image height. Consequently, h.sub.1 can be used as quantitative scale for comparing the size of the zoom lenses, particularly the diameters of the lens of the first group or that of the filter to be mounted before the lens.
TABLE 3 ______________________________________ -h.sub.1 w -h.sub.1 t .SIGMA.1/fi ______________________________________ Zoom Lenses (Table 1) 5.4766 6.2680 -0.004177 Zoom Lenses (Table 2) 4.2927 5.2318 -0.014625 ______________________________________
Table 3 shows h.sub.1 w in the extreme wide angle position, h.sub.1 t in the extreme telephoto position and .SIGMA.1/fi. The values h.sub.1 of the lens in accordance with Table 2 are about 0.8 times as large as those of the lens in accordance with Table 1. The diameter of the first group is also about 0.8 time as large when the focal length of the respective groups of the zoom part and the distance between the principal points of the groups are proportionally decreased. On the other hand, .SIGMA.1/fi of the lens in accordance with Table 2 is larger in absolute value, which means the curvature of field is over-corrected in comparison with the lens in accordance with Table 1 in such a manner that it is difficult to take a balance between the spherical aberration and the curvature of field.